Manufacture of ultramarine



and was seriously deficient in Patented May 25, 1948 h MANUFACTURE OFULTRAMARINE Ailing P. Beardsley, Plainfield, and Stanley H.

Whiting, Martinsviile, N. J assignors to American Cyanamid Company, NewYork, N. Y., a corporation of Maine No Drawing. Application July 24,1945,

' Serial No. 606,888

and Germany from 1828 until about 1850-60,

were these-called sulfate and suifate-soda processes. In these processesa mixcontaining calcined clay, sodium sulfate, and charcoal, in the caseof the sulfate process; and clay; sodiumsulfate,-sodiurn carbonate,sulfur and pitch in the case of the sulfate-soda-process, were'fired toproduce a pigment which was green, and not blue.

This pigment, known as ultramarine green, was sold as such as a greenpigment, and ifit was desired to produce-ultramarine blue the greenpigment was oxidized in a separate step, b Washing it free from sodiumsulfate, drying, preferably mixing with about 5% of sulfur,- and thenfiring on a hearth for about a day and cooling off for several days.This second step produced a -blue.ultramar ine which, however, wasnot-of the type sold today as it had a relatively low sulfur content,below 10%, and usually about 5%, strength by modern standards.

After 1860 most plants changed toa process whichwas carried outphysically in a single furnace. A mix containingabout equal amounts ofkaolin, soda ash and sulfur, with additional silica, and from -10 to 12of the weight of the soda ash inrosin or pitch, was loaded intocrucibles-havn a definite porosity; These crucibles were then subjectedto firing for a period of about two days and permitted to coolfor a longperiod of from ten days to as muchas three weeks. During the veryslowcooling oxidation took place. This operation in a singlefurnaceproduced an ultramarine which was muchhigher in sulfur than theformer type, ranging-from an-exceptional minimum o'f l0% up to a normalof 12 to 14%. 7 Also, .it wasnoted that this process, if based on aoperation process, is still in use today and it is ioiaims. (01.106-305) 2 the basis of modern secondary ultramarine production. 1 I

The oxidation step which increases'the strength and brightness of theblue greatly does not have any effect, detectable by chemical analysis,except the removal of from about /1 to A; of the sodium in the crudeultramarine, which sodium is transformed into sodium sulfate.

The single operation process using kaolin,si1ica, soda, sulfur andhydrocarbon has many very serious drawbacks. One is that the secondaryultramarine is not uniform in strength. Even when the greatest pains aretaken to attemptto fill each crucible to exactly the same density,provide for identical porosity, and carry out the firing and coolingprocedure in exactly the same manner, no two batches of ultramarine comeout alike and a, good deal of material is of low strength and has to besacrificed or sold 'at a much lower price as inferior gradeultrarnarine.

A second disadvantage, which might be con sidered acorollary ofthefirst, thatthe shade of-the ultramarine willfvary from furnace tofurnace, from one part of a batch to another, and even in differentparts of a singlecrucible. Sometimes the general shade comes outredeblue, and then again greenish-blue, and the problem of producinguniformity of shade has never been solved. Ultramarine manufacturershave to select from a large number of batches to obtain uniformproducts. This requires developing sales for many different types ofblue,,a larger inventory because aseriesof'batches may run to anundesired shade, and it is necessary to wait until adesired shade isproduced. j A' further disadvantage of the ordinary commercialprocess'es'jtoday lies in the fact that the output from equipment isvery low. The time cycle is excessive. In ,mostcases from raw mix tocooled, fur'naced secondary ultramarine from 15-to 20 days must elapse,Ultramarine furnaces represent a considerable capital investment and thefixed chargesare therefore high. V According to the present invention wehave found that the requirements for the two stages in which the.ultramarine reaction necessarily proceeds are entirely'diiferent, andthat the single operation process is a compromise in which neither stageiscarried out under satisfactory conditions. Therefore, according to thepresent invention the ultramarine is produced in two separate stepsunder radically different reaction conditions. We'h'ave foundthat thechemically active constituents of furnace gases are injurious if-theycome into contact with-themixture in the first stage of reaction whichproduces the weak, dull, primary ultramarine blue. Water vapor adverselyaffects the product, oxygen is particularly deleterious, and at acertain stage, carbon dioxide is also harmful. Therefore, the first stepof the two step process er the present invention is "carried out in amanner so that be a matter of much importance, and one of active furnacegases, and particularly oxygen,

are kept from the charge. In a more specific aspect of the presentinvention this first step is carried out in non-porousceramiccrucibles.v It

is possible, however, to use crucibles some. porosity, such as thosewhich showa'watfer ab;

sorption of less than %{ahd' preferably not more than 5%, where thecrucibles are of such .some diificulty, as the reaction of oxidation isexothermic either by; air or by sulfur dioxide. In detail, thisoxidation is complex and not completely understood, Certainly theprimary ultra- ,marine is converted to the secondary ultramarine,;an'dthe sodium polysulfides resulting from .the 'firingstep are oxidized to,sodium sulfate.

size that the gases evolved froi'nthe reictin'" charge maintainsufiicient pressure, including partial pressure of sulfur vapor, toeffectively oppose entry of the furnace gases or at least to reduce suchentry to njejgligible proportions. The amount of damage with a, givenporosity is, hf course, influenced by ..the size of the erucibl'e,because the ratio of wall area t'o'volinne :of eharge dehrease-s withthe dimensions offthe cri'icible.

For a practical crucible of at least nine inches in diameter. the lossesare not serious if the porosity, as measur'ed'by water a scrpuen, .doe'snot exceed 5 and therefore slightly porous crucibles are included in thefirst. step in the broader aspects'of the present invention.

The first step of the present, process may, of course, be carried. outwith the use or containers of considerably larger diameter than nineinches, but thed-imcult'ies of "g'ettingheatrapi-dly' to the interiorwithout too great temperature gradient increase rapidly with the"d'ia'irreter.

, The ideal conditions for the second step, produot ion ofsecondary.uitiqamanneas we have learned them, are very difiereritIromthose for thefirst. They comprisetwecentrcis I p d; 'ccmmze-ofi;idaiio'1't.-Wheifeas in thefirst stage any accessof oxidizing.agents the. charge is harmnirin the se ehdistage Ibis a necessity.

However. if air is used, as, the oxidizing agent, its access mustbcontrolled, too free access will over-oxidize the ultramarihe ior'ining'vamei'ess white inaterial. A "way to control the access of oxygen is tocharge the prodfi'ot 'o'f'the first stage into porousoruoibles. Sucherueib'res can suitably have 'poresity or about 16% or higher, asmeasured by water absorption.

Another-,and evenbetter way to control the oxidation, and the way whichwe prefer, is to use sulfur dioxide in place of air. This use er sulfurdioxide is disclosed and elaimed our copen'ding application No. 606,887,filed July 24, 1945. We

fi lld that sulfur dioxide hasthe power to oxidize ultrainarine of theprimary stage to uu'rain anne ofthe secondary stage but not toover-oxidize it to white. Since the danger of over-oxidation does notexist, thereis no need to control the rate of access of sulfur dioxide,except 'as limited by development of temperature, as described below.For oxidization by sulfur dioxide, the primary'stage ultran arineinay beplaced in nonporous containers, into which sulfur dioxide is fed. Theuse of sulfur dioxide for oxidizing primary ultramarine to secondaryultramarine is not claimed broadly in this application.

19. Control of temperature.-We have found that the temperature at whichprimary ultramarine is oxidized has a great effect upon the shade of theproduct. If oxidization occurs at and is unsuitable for most purposes.Between 5 which must be removed if the temperature is to becontrolled.

\ In order to control the temperature during the oxidation period withinthe zone of 200-550f C. (and preferably at eteut 4007-55 0 C); We findit desirable to provide apparatus for the "second stage in whichheat canbe first applied tothe blue of the primary stage to raise it; t'cfthedesired temperatu eror oxidation, and then heat can be withdrawn fromthe charge as the oxidation develops heat within. Z l

g It is clear from the above that the present single operation processis acompromise. The crucibles, m orurte be porous eneugh for theoxidation step, have to be too porous for the firing step. Thetemperatur during oxidation follows the whole range cewnwara from thetop iirin'g temperature of 750 (in many parts of the furnace 850 orhigher) to substantially atmospheric. we believethat in this wide'r'angeor'temper'ature of oxidation in the pre ent process lies one o'f theprincipal causes of the 'unpreaictame variations in shade 'foundby theultramarine manufacturer. However, we do not wish to limit our inventionto any theory of reactionf In, the present process no compromise isnecessary. The oxidation'st'ep can be carried out in containers mostsuited to it without regard to therequirements c'rthefir'st or firingstep. The long time for oxidation which is dictated by the necessity forlow porosity or'u'cibles in the first step is not encountered. This isparticularly true when sulfur dioxide is used as the oxidizing gas. 7 VV A particularly important factor in the present invention is that theoxidation step can be carried out under exact temperature control. It isno longer necessary to permit the ultramarine crucibles to co'ol at therate dictated by the 'furnacecharacteristics. This is of primeimportance, because over-oxidation 'which resul'ts in loss of color,finally producing a white or grayish product, is'rn'uch more dangerousat high temperatures and excessively high temperatures :are also likelyto produce a blackish secondary blue.

In the ordinary commercial process the ultramarine manufacturer has nocontrol. His crucibles cool down continuously and he cannot keep them atexactly the best temperature for the nec- 'essary time for the desiredtype of Oxidation.

When, however, this step is carried out indifferent apparatus theconditions can be accurately controlled and 'no compromise is necessary,7 In addition to producing more uniform, higher quality secondaryiiltramarine in better yield, thetwostep process of the presentinvention also effects a marked saving in time, with a correspondinglygreater output from a given size of equipment.

The increase in strength obtainable .bythe twostep process of thepresent invention is quite'st'art- Comparing the ultramarines ling. Itis possible with careful operation to produce an ultramarine having 75%greater strength than the normal commercial 'ultramarineproduced by thestandard one-step process in commerce used today. Atthe same time thisgreat increase in strength is obtained without additional productioncosts, as any additional handling is offset by the shortened time cycle.a

The preferred modification or the resent in- Vention involves the useofall. impervious ceramic material which, preferably, should also show alow coefficient of expansion with temperature and a high coefiicient ofheat conductivity. Fused silica is an excellent product on account-oiits'low coeflicient of expansion. Other impervious ceramics whichexhibit the characteristics mentioned, such as those containing siliconcarbide, may also be used. The ceramic ware must withstand fairly fastheating rates, temperature rises of about 100 C. per hour or more beingdesired to obtain the most economical throughput.

The use of impervious ceramic containers, which constitutes thepreferred modification of the present invention, should not be confusedwith the use of brick lined metal containers. Such a procedure wasproposed in 1891. It was never practical and the reasons are obvious.The refractory lining and the metal have difierent coefiicients ofexpansion, and as soon as a crack forms sulfur vapor, which is alwayspresent in the first heating stage, rapidly attacks the metal causing itto swell which still further cracks the lining. In the present inventionthe high shock-resisting ceramic ware prevents contact of deleteriousgases with the ultramarine mix, and as it is free from metal nodifficulties are encountered with sulfur corrosion or contamination ofthe product with undesirable metal sulfides.

The preferred embodiment of the two-stage process involves the transferof the product of the first stage from its containers to othercontainers in which the second stage can be more suitably carried out.This requirement-the transfer of the charge-can be most conveniently metif the original mix is converted into briquet form before it is chargedinto containers for the first stage. The conversion of the rawmix intobriquets is disclosed in our copending application No. 606,886, filedJuly 24, 1945. This improvement is not claimed, per se in the presentapplication.

Throughout the specification and claims the term "primary ultramarinewill be used to cover the product obtained in the first firing stage upto about 750 C. before oxidation. The ultramarine produced in the firststage and which is referred to as primary ultramarine is a blue pigmentalthough dull and of no strength. The contents of a crucible appeargreen because the blue primary ultramarine is mixed physically withyellow sodium polysulfides formed in the firing. Similarly in thespecification and claims the terms secondary ultramarine will be used tocover the blue pigment obtained by oxidation of the primary ultramarine.The two terms above defined will be used in no other sense.

The term high strength, high sulfur ultramarine will be used in theclaims in its customary sense to mean modern ultramarine of high colorvalue having a sulfur content of at least The invention will bedescribed in detail in connection with a typical operation under thetwostep process of the present invention. The parts are by weight.

was thoroughly ground together in a pebble mill.

The mixture was'packed into fusedsilica crucibles z-andifired :in afurnace whose temperature was raised 50 C. per hour to a temperature of700 C.'; wherei twas held from 4 to 8 hours.

The crucibles were cooled, the green-colored contents removed to porousalundum cylinders, the cylinders well sealed and inserted into a furnacewhose temperature was 350 0. While the temperature was held at 350, airwas circulated slowly through the furnace and around the cylinders tillthe percent of oxygen in the exit air equalled that in the inlet air,and then for 2 hours more. After cooling, the ultramarine was washed,wet-ground in a pebble mill loaded with about pebbles, dried and tested.The strength averaged of th strength of the same composition of mix,fired and oxidized in plant furnaces in the usual single operation.

This was ground in a pebble mill containing pebbles of about dia. Theground mix was briquetted cold by pressure, the briquets placed in animpervious fused silica container, heated to a temperature of 720 C.during six hours and held there four hours. After cooling, the briquetswere removed, cracked, and transferred to another fused silica containerin which they were heated to a temperature of about 420 while sulfurdioxide was supplied to them. After oxidation was completed, the bluewas washed, wet-ground in a pebble mill with pebbles of about /4 todia., and tested for strength. The strength was found to be 200% of theaverage blue of the highest strength being made in the factory at thesame period in the usual single operation.

It will be noted that when the two-step process is used with sulfurdioxide as the oxidizing agent, the results are even better than inExample I 'where the oxidation was effected with oxygen.

We claim:

1. In a two-step process for producing high strength, high sulfursecondary ultramarine" in which a raw, high sulfur ultramarine charge isfired at a reaction temperature of not more than about 750 C. to formprimary ultramarine followed by controlled oxidation of the primaryultramarine to secondary ultramarine, the improvement which comprisesmaintaining the charge in the first step substantially free fromchemically reactive gases, cooling the fired charge, and carrying outthe oxidation of the primary ultramarine in a separate step at atemperature of about 200-550 C.

2. A method according to claim 1, in which the oxidation of primaryultramarine to sec- 7 ondary ultramarine is effected at a substantiallyconstant elevated temperature, cooling being employed at the beginningof the controlled reaction until the sulfides present in the primaryultramarine are Substantially oxidized.

3. The process according to claim 1 in which cooling is applied duringoxidation of residual sulfides until approximately to of the combinedsodium in the primary ultramarine has been converted to sodium sulfate.

4; A method according to claim 1 in which the oxidation of primaryultramarine to secondary ultramarine is effected at a temperature from400 to 550 C.

ALLING P. BEARDSLEY. STANLEY H. WHITING.

, M H REFERENCES cn n" 7 1" .The following references are of record inthe file of this patent:

OTHER REFERENCES Manufacture of Colors for Painting, by Riffault,Vergnaud, Toussaint, 1874, pages 304, 305,

1 15 311, and 3 9

